Again, another group buy offers absolutely same rules and reinvestment details as your group buy. These shares worth same as your group buy shares. But they offer 150 GHs for 30$ at start (for comparison to your 110 GHs 30$). They can make it, because they don't have old investors and have access to new hardware.

Then, their R2 shares will rise to 200 GHs, while your R4 shares will rise to 155 GHs.
Then, their R3 shares will rise to 300 GHs, while your R5 shares will rise to 250 GHs. Another company pop up with 30$ for 350 GHs shares.

It's just the example, but you should get the point. These problems you could meet in future and come to the point where you couldn't open new rounds.